1. Field of the invention
This invention relates to an exhaust gas purifying system for use in an internal combustion engine and more particularly to an exhaust gas purifying system of the type described which includes a catalyst convertor, an air injection means for oxidizing HC and CO by injecting air into an exhaust manifold and/or an exhaust gas re-circulating means for suppressing the generation of NO by returning part of the exhaust gases to a combustion chamber.
2. Description of the prior art
Exhaust gas test under the running conditions specified in U.S. 10 mode or 11 mode is known, by which to measure the quantity of exhausted unburnt toxic gas components contained in exhaust gases from an engine of an automobile which is typical of internal combustion engines. According to test, an automobile is driven so as to effect an idle, accelerating, constant speed and decelerating runnings, respectively, which are specified according to said mode, before and after the engine warm-up running, whereby the quantity of the toxic unburnt gas components contained in exhaust gases from an automobile during its running is measured and tested for its allowance.
Known as a countermeasure for reducing the quantity of unburnt gases from an automobile engine, i.e., so called exhaust gas countermeasures, are the one which is referred to as an engine modifying method, by which to vary, for instance, the combustion conditions of an engine and the one which is referred to as an exhaust gas post-treating method, by which to utilize chemical reactions such as oxidation and reduction for exhaust gases from cylinders of an engine or to utilize physical reactions such as filtration and adsorption of exhaust gases.
Disclosed as modifications of a suction system according to said engine modifying methods are (i) a choke opener which releases the actuation of a choke valve adapted to supply a relatively richer mixture gas to cylinders in an attempt to improve the running performances of an engine at the time of cold running, thereby rapidly providing a normal air-fuel ratio for the mixture gas to be fed, (ii) an auxiliary accelerating pump for preventing the decrease in the running performances of an engine which is caused by the operation of said choke opener, (iii) a throttle positioner for preventing the decrease in the compression ratio of a mixture gas within cylinders by preventing the rapid shifting of a throttle to its closed position at the time of deceleration and (iv) a fast idle device adapted to vary the idle opening position of a throttle valve before and after the warm-up running of an engine. In addition, known as modifications of an ignition system is a vacuum ignition advancer for controlling the ignition timing at the time of a normal running. The above-enumerated devices, in general, are operated due to a negative pressure within a suction pipe and prevent the discharge of harmful unburnt gas components by bringing the combustion of a mixture gas in cylinders to an improved condition.
Known as the measures according to the aforesaid exhaust gas post-treating method are (i) an air injection device which feeds the air from an air cleaner to an exhaust manifold to thereby cause the reaction of unburnt gas components, (ii) a catalyst convertor which causes oxidation of unburnt gas components contained in exhaust gases by using catalysts, and (iii) an exhaust gas re-circulating device which lowers a combustion temperature within cylinders by returning part of exhaust gases to a suction pipe. Said air injection device and catalyst convertor prevent the discharge of unburnt gas components such as HC and CO by resorting to the oxidation reaction, while said exhaust gas re-circulating device lowers the combustion temperature in cylinders to thereby prevent the generation of NO for purifying exhaust gases.
According to one of the conventional exhaust gas purifying systems for reducing the quantity of unburnt gases exhausted under the running condition specified in said mode below the allowance, there are provided a choke opener, a throttle positioner, a fast idle device, a vacuum ignition advancer, means for injecting air to an exhaust manifold, a catalyst convertor and an exhaust gas re-circulating device. However, since the air injection to said exhaust manifold accompanies a high temperature due to the oxidizing reaction, it is mandatory to stop the feed of air for said air injection from viewpoints of running performance, safety and protection of the catalyst convertor, at the time of high loading running as well as at the time of starting a decelerating running. This is particularly true in the decelerating phase of an engine which is likely to incur a misfire to an engine, particularly in the case of starting decelerating running of an engine which is most likely to cause a misfire. More particularly, it is imperative to stop the feed of air at the time of an high speed running which requires a high R.P.M. for an engine, at a throttle-full-open running which imposes a high load on an engine, at the time of engine brake running for a long period of time and at the time of running when an engine is subjected to a high temperature, for the purposes of preventing an afterfire as well as for preventing overheating of a catalyst convertor as well as deterioration of catalysts due to said overheating.
On the other hand, the feedback of said exhaust gases to a suction pipe by means of said exhaust gas re-circulating device will lead to the decrease in an output of an engine, because the combustion temperature is lowered. It follows from this that, at the time of high loading running, at the time of low temperature running of an engine, at throttle-valve-full-open running and at the time of decelerating running, the feedback of exhaust gases to the suction pipe should be stopped. More specifically, at the time of the throttle valve-full-open, high speed, high loading running which requires a considerably high output of an engine, and at the time of idle running at a low level of an output of an engine or at the time of low temperature running of an engine, the feedback of the exhaust gases to the suction pipe should be stopped for preventing the decrease in the output of an engine, while at the time of decelerating running, the feedback of exhaust gases should be stopped for preventing an unsatisfactory combustion condition of a mixture gas in cylinders as well as poor ignitability of a mixture gas due to the re-circulation of exhaust gases.
For those reasons, said conventional exhaust gas purifying system is provided with (i) an air switching device for feeding the air from an air cleaner to an exhaust manifold commensurate to the running condition of an engine as well as for returning air to the air cleaner, when not required, (ii) a shut-off valve for interrupting exhaust gases from being returned from the exhaust manifold to the suction pipe, and (iii) a device for actuating said shut-off valve.
With the conventional air switching device, the intelligence as to the running conditions such as a temperature at an engine which is fed from a vehicle speed sensor and the like is collected and fed to a computor for analysis, after which according to a signal from the computor, the air from an air cleaner is fed to an exhaust manifold commensurate to the running conditions or the air is returned to said air cleaner, while said shut-off valve actuating device returns exhaust gases to the suction pipe according to signals from the computor, as in said air switching device, or prevents the feedback of exhaust gases.
As is clear from the foregoing, the conventional exhaust gas purifying system dictates the provision of a computor for feeding signals so as to operate the air switching device and shut-off valve actuating device, thus resulting in a complicated construction and hence high cost. Another disadvantage is that there is a possibility of causing a vehicle fire and overheating of a catalyst convertor which is one of the causes for deterioration of catalysts due to the increasingly vigorous reaction within the catalyst convertor, which reaction is caused by the increase in the quantity of unburnt gases contained in exhaust gases, at the time of an engine-brake running for a long period of time.